Polymers of styrene have long been known as useful, and in particular, they are known to be useful as block copolymers with conjugated diolefins, such as butadiene and isoprene. These block copolymers are referred to as thermoplastic polymers because they may be worked by heating the polymer to its melting temperature, and then processing by such methods as vacuum forming, extrusion, compression molding, calendering or blow molding. When the styrene content of these polymers is less than about 60 percent by weight, these polymers are also elastic. As elastomers they have good tear resistance, flexibility, thermomechanical stability and other properties.
The thermoplastic properties are the result of incompatibility between the polystyrene and the polydiolefin polymeric blocks which causes separate domains to exist. At service temperatures, the polydiolefin domains are rubbery and elastic, whereas the polystyrene domains are hard and glassy. The polystyrene domains serve as physical crosslinks between the rubbery polydiolefin blocks. This causes the polymer to behave much like a vulcanized rubber at temperatures which are below the polystyrene glass transition temperature. By heating the block copolymer to a temperature greater than the glass transition temperature of the polystyrene domains, the polymer may be processed as a melt and processed.
Although these polymers are processable as a melt formable thermoplastic in commercial polymer handling equipment and have many excellent properties, they have shortcomings. The glass transition temperature of the polystyrene domains limit the polymer's service temperature. Further, these polymers are not compatible with polar engineering thermoplastics, causing difficulty in making polymeric blends with these materials. Adhesion to polar substrates and polar coatings are also generally not good, and organic solvents readily dissolve these polymers.
It has been found that these shortcomings can be partially overcome by incorporating polar functional groups into the polymers. It has been found to be particularly advantageous to incorporate salt functional groups into the polystyrene blocks to increase the polymer's solvent resistance, high temperature properties and tensile strength. Polymers which include salt functionality are commonly refered to as ionomers. Incorporation of salt of sulfonic acid functionality into the styrene blocks of such a polymer may increase the service temperature to 200.degree. C. Incorporation of salts of carbonic acid functionality into the polymers may increase ultimate use temperatures to above 125.degree. C. The same polymer without functionality is limited to service temperatures of about 100.degree. C.
Unfortunately, incorporation of salt functionality into these polymers is detrimental to the processability of these polymers. The ionic interactions which cause the improvements in the polymer's mechanical properties at service temperatures remain active above the glass transition temperature of the polystyrene domains. These ionic interactions interfere with the processability of the polymer melts by raising the viscosities of the melt. Typical plasticizers, such as processing oils, esters and glycerols, improve processability of styrene block functionalized styrene-butadiene block copolymers, but only at the expense of many of the physical properties which the functionalization is intended to improve.
Other ionomers which are difficult to process are also known, and plasticers for some of them have been developed. Terpolymers of ethylene-propylene and diolefins, such as EPDM, which are sulfonated and then neutralized with metal ions to produce salt functionality have been known since the early 1970's. The ionic crosslinks of these polymers serve many of the same functions as the styrene blocks of the styrene-conjugated diene block copolymers. The ionic crosslinks tie polymer chains together to result in properties like vulcanized rubber, but the ionic crosslinks can be broken at elevated temperatures with the aid of a plasticizer. Preferred plasticizers for salts of sulfonated EPDM include zinc stearate and aliphatic organic amides. These plasticizers are taught in U.S. Pat. No. 3,847,854. They are referred to as ionic plasticizers because they function by relaxing ionic bonds. This is in contrast to backbone plastization which is accomplished by enhancing slippage between polymeric backbones.
Zinc stearate can be utilized as a processing aid for styrene functionalized block copolymers of styrene and butadiene, but the physical properties of the functionalized block copolymers, and in particular, physical properties near the polystyrene domain glass transition temperatures are significantly adversely effected because the zinc stearate lowers the glass transition temperature of these domains.
Another family of plasticizers for salts of sulfonated polymers operates by releasing water at processing temperatures. Examples include CaSO.sub.4 .multidot.2H.sub.2 O, LiSO.sub.3 .multidot.2H.sub.2 O, (NH.sub.4).sub.2 SO.sub.4 .multidot.Ce(SO.sub.4).sub.3 .multidot.8H.sub.2 O, BiO.sub.2 .multidot.2H.sub.2 O and FeF.sub.2 .multidot.8H.sub.2 O. Processing aids which release water at elevated temperatures are also taught in U.S. Pat. No. 3,847,854. The water released from these plasticizers is retained in the polymer melt while the polymer is under a high pressure, such as in an extruder. When a polymer melt exits a region of high pressure, the water is released from the melt. The use of this family of plasticizers therefore does not allow reprocessing without addition of more plasticizer.
Other known plasticizers for functionalized elastomers include dioctyl phthalate (DOP), dioctyl sebacate, dibutyl phthalate (DBP), diethyl phthalate (DEP), and dioctyl succinate. These plasticizers are liquids at room temperature. When incorporated into compositions of ionomers of polystyrene they cause the glass transition temperature to decrease and therefore result in a deterioration of high temperature mechanical properties. Low molecular weight amines are also known as processing aids for ionomers, but the use of low molecular weight amines also causes the glass tranition temperature of ionmerized polystyrene to decrease.
It is therefore an objective of the present invention to provide a plasticizer for functionalized monovinyl aromatic polymers which does not significantly reduce the high temperature properties of the polymer. In another aspect, it is an objective of this invention to provide a composition comprising such a polymer and a plasticizer which may be processed at temperatures typical of commercial polymer processing equipment.